This invention relates to variable speed hoisting driven by an induction motor for hoisting a suspended load, and more particularly to protection control circuits for the induction motor thereof.
FIG. 5 is a circuit diagram showing a conventional protection control circuit for a variable speed hoisting device, which is disclosed, for example, in Japanese Laid-Open Patent (Kokai) No. 62-46879. An inverter unit 1 is supplied with a three-phase alternating current from current supply lines R, S, and T. A hoisting induction motor 2 is supplied with the three-phase alternating current from the inverter unit 1 and rotates in a direction and at a speed corresponding to the phase rotational direction and the frequency, respectively, of the three-phase alternating current supplied thereto. A break coil 3 activates a break to lock the induction motor 2, when the current supply thereto stops. The break coil 3 releases the break to allow a free rotation of the induction motor 2, when current is supplied thereto.
When a normally open push button switch for upward operation PBu is pushed to the first level, the first contact PBul is closed. When it is pushed to the second level, the second contact PBuh is closed. When a normally open push button switch for downward operation PBd is pushed to the first level, the first contact PBdl is closed. When it is pushed to the second level, the second contact PBdh is closed. By the way, the normally open push button switch for upward operation PBu and the normally open push button switch for downward operation PBd are mechanically interlocked such that they cannot be pushed in simultaneously.
When excited, an electromagnetic relay coil 4 closes the normally open contacts 4a1 and 4a2. When the normally open contact 4a1 is closed, the inverter unit 1 supplies a three-phase alternating current upward hoisting direction. Similarly, when excited, an electromagnetic relay coil 5 closes the normally open contact 5a1 and the normally open contact 5a2. When the normally open contact 5a1 is closed, the inverter unit 1 outputs a three-phase alternating current in the downward hoisting direction. When the inverter unit 1 is outputting the three-phase alternating current, an electromagnetic relay coil 6 is excited to close a normally open contact 6a1. Further, when excited, an electromagnetic relay coil 7 closes normally open contacts 7a1 and 7a2 to supply current to the break coil 3 such that the break for stopping the induction motor 2 is released.
A high-speed level setter 8 and a low-speed level setter 9 consist of a variable resistor respectively. An electromagnetic relay coil 11 is excited when either the normally open push button switch for upward operation PBu or the normally open push button switch for downward operation PBd is pushed to the second level. Then, the normally open contacts 11a1 and 11a2 are closed and the normally closed contacts 11b1 and 11b2 are opened, so as to change over the speed setting from that by the low-speed level setter 9 to that by the high-speed level setter 8.
When excited, an electromagnetic relay coil 13 closes the normally open contacts 13a1 and 13a2. The normally open contact 13a1 is a self-keeping contact which is kept closed once the electromagnetic relay coil 13 is excited by the closure of the normally open contact 5a2. When the normally open contact 13a2 is closed together with the normally open contact 4a1, the inverter unit 1 outputs a three-phase alternating current to the induction motor 2 in the phase rotational direction of upward hoisting operation. A rotational speed indicating generator 14 (rotation detector) generates a DC voltage having a voltage level and a polarity corresponding to the rotational speed and rotational direction, respectively, of the induction motor 2. An abnormality detector 15 is supplied with the outputs of the rotational speed indicating generator 14 and the normally open contact 4a2. Assume that, in spite of the fact that the normally open contact 4a2 is closed (namely, the upward hoisting operation is instructed by the operator), the polarity of the DC voltage of the rotational speed indicating generator 14 corresponds to the downward hoisting direction of the induction motor 2. Then, the abnormality detector 15 judges that the suspended load which should be hoisted up is falling abnormally, and thus opens the normally closed contact 15b1, thereby stopping the upward hoisting operation as described below.
Next the method of operation of the protection control circuit for the variable speed hoisting device of FIG. 5 is described by referring to FIG. 6.
FIG. 6 is a diagram showing the variation of the output frequency of the inverter of the variable speed hoisting device. When the current supply lines R, S, and T are connected to the respective power source lines, the electromagnetic relay coil 13 is not in the self-keeping state. When, however, the normally open push button switch for downward operation PBd is pushed in, the electromagnetic relay coil 5 is excited to close the normally open contact 5a2. Then, since the normally closed contact 15b1 of the abnormality detector 15 is closed, the electromagnetic relay coil 13 is excited to close the normally open contact 13a1, to be retained in the self-keeping state. Then, even when the normally open push button switch for downward operation PBd is released and the normally open contact 5a2 is opened, the normally open contacts 13a1 and 13a2 are kept closed.
When, under this circumstance, the normally open push button switch for upward operation PBu is pushed to the first level at time point t.sub.1, the normally open contact 4a1 of the electromagnetic relay coil 4 is closed. Since the normally open contact 13a2 is already closed by the self-keeping electromagnetic relay coil 13, a signal indicating an instruction for an upward hoisting operation is input to the inverter unit 1. Thus, the frequency of the inverter unit 1 begins to increase at a predetermined rate from 0 Hz to f.sub.2 Hz set by the low-speed level setter 9. When the frequency reaches at f.sub.1 Hz at time point t.sub.2, the inverter unit 1, detecting the frequency by means of a built-in frequency detector (not shown), starts to supply the three-phase alternating current to the induction motor 2 and, simultaneously therewith, excites the electromagnetic relay coil 6. As a result, the normally open contact 6a1 is closed and the electromagnetic relay coil 7 is excited, thereby closing the normally open contacts 7a1 and 7a2. Thus, the break coil 3 is supplied with a current, and releases the break of the induction motor 2. The induction motor 2 thereupon starts to rotate in the upward hoisting operation and gradually gains speed.
When the frequency of the inverter unit 1 reaches a f.sub.2 Hz set by the low-speed level setter 9 at time point t.sub.3, the inverter unit 1 stops increasing the frequency thereof. Thus, the induction motor 2 is operated at a fixed low speed. When the normally open push button switch for upward operation PBu is pushed to the second level at time point t.sub.4, the electromagnetic relay coil 11 is excited to close the normally open contacts 11a1 and 11a2 and to open the normally closed contacts 11b1 and 11b2. Thus, the low-speed level setter 9 is separated from the inverter unit 1, while the high-speed level setter 8 is connected thereto. As a result the inverter unit 1 starts to increase the frequency at a predetermined rate, from f.sub.2 Hz set by the low-speed level setter 9 to f.sub.3 Hz set by the high-speed level setter 8. Thus the induction motor 2 again accelerates.
When the frequency reaches at f.sub.3 Hz at time point t.sub.5, the inverter unit 1 stops increasing the frequency thereof. Thus, the induction motor 2 is operated at a fixed high speed. Thereafter, when the normally open push button switch for upward operation PBu is returned from the second to the first level at time point t.sub.6, the speed setting is changed over from that by the high-speed level setter 8 to that by the low-speed level setter 9. As a result, the frequency of the inverter unit 1 begins to decrease at a predetermined rate. When the frequency reaches at f.sub.2 Hz at time point t.sub.7, the inverter unit 1 stops decreasing the frequency thereof. Thus the induction motor 2 is again operated at a fixed low speed.
Further, when the operator completely ceases to push the normally open push button switch for upward operation PBu, the electromagnetic relay coil 4 ceases to be excited and the normally open contact 4a1 is opened. As a result, the frequency of the inverter unit 1 begins to decrease toward 0 Hz at a predetermined rate. When the frequency reaches at f.sub.1 Hz at time point t.sub.9, the inverter unit 1 stops supplying the induction motor 2 and exciting the electromagnetic relay coil 6, thereby opening the normally open contact 6a1. As a result, the excitation of the electromagnetic relay coil 7 ceases, and the normally open contacts 7a1 and 7a2 are opened. Thus, the supply of current to the break coil 3 is stopped, and the break coil 3 activates the break to stop and lock the induction motor 2.
The downward hoisting operation which is initiated by pushing the normally open push button switch for downward operation PBd is similar to the upward hoisting operation described above. In the case of the downward hoisting operation, however, the electromagnetic relay coil 5 is excited to close the normally open contact 5a1, such that the induction motor 2 is rotated in the downward hoisting direction.
During the upward hoisting operation, the abnormality detector 15 operates as follows. As described above, when the electromagnetic relay coil 13 is self-keeping, the normally open contact 13a1 is closed and the upward hoisting operation can be performed. Assume that a suspended load which exceeds the rating of the hoisting device is suspended and the normally open push button switch for upward operation PBu is pushed to the first level. Then the induction motor 2 tries to start the upward hoisting operation. However, the torque of the induction motor 2 is insufficient to hoist the load, and the suspended load begins to fall. Then, the rotational speed indicating generator 14 outputs a voltage whose polarity corresponds to the downward hoisting direction. The normally open contact 4a2, however, is closed at this time, to inform the abnormality detector 15 that the upward hoisting operation is being performed. Thus, the abnormality detector 15 judges that the suspended load which should be hoisted up is abnormally failing, and hence opens the normally closed contact 15b1. As a result, the self-keeping action of the electromagnetic relay coil 13 is broken, and the normally open contact 13a2 is opened. In response thereto, the inverter unit 1 stops the upward hoisting operation.
When the induction motor 2 is thus stopped, the output from the rotational speed indicating generator 14 ceases and thus the abnormality detector 15 closes the normally closed contact 15b1. When the normally open push button switch for downward operation PBd is again pushed in, the electromagnetic relay coil 5 is excited to close the normally open contacts 5a1 and 5a2. As a result, the electromagnetic relay coil 13 is again brought into the self-keeping state, and the normally open contact 13a2 is closed. Thus, the upward hoisting operation again becomes feasible.
The above conventional variable speed hoisting device, however, has the following disadvantage. When the rotational speed indicating generator 14 stops generating its output due to a failure thereof or a disconnection from the abnormality detector 15, etc., the abnormality detector 15 becomes incapable of detecting the abnormal operation of the hoisting device such as the abnormal fall of the suspended load.